Recently, there have widely been employed electric power steering apparatus, which detect a steering torque generated by a steering handle with a steering torque sensor, and control a motor to generate a steering assistive force depending on the detected steering torque, for thereby enabling the vehicle to be turned with a light steering force applied to the steering handle.
Such electric power steering apparatus perform a phase compensation on the steering torque, which is detected by the steering torque sensor, in order to improve the steering sensation experienced by the driver.
As shown in FIG. 6 of the accompanying drawings, Japanese Laid-Open Patent Publication No. 2004-098754 (JP2004-098754A) discloses in paragraph [0010] and FIG. 4 thereof an electric power steering apparatus, which performs a phase compensation of the type described above. According to the disclosed electric power steering apparatus, a steering torque, which is applied to a steering wheel 201, is transmitted through a steering shaft 202 (an input shaft 202A and an output shaft 202B) to a steering mechanism 203. At this time, a drive force generated by an electric motor M is mechanically transmitted to the steering mechanism 203 as a steering assistive force.
The input shaft 202A and the output shaft 202B are coupled to each other by a torsion bar 204. The direction and magnitude of a twist in the torsion bar 204 is detected as a steering torque signal T by a torque sensor 205.
Using a target current setter 212, a controller 210 sets a target current value, which depends on the steering torque signal detected by the torque sensor 205 and a vehicle speed V detected by a vehicle speed sensor 206. The controller 210 controls a motor driver 220 based on the set target current value, so as to impart a drive current to the electric motor M, which applies a steering assistive force to the steering mechanism 203 that depends on the steering torque signal T and the vehicle speed V.
The controller 210 includes a phase compensation processor 211 for performing a phase compensation process in order to adjust the gain and phase of the steering torque signal T from the torque sensor 205, and the target current setter 212 for setting a target current value, which depends on the steering torque signal T that is phase-compensated by the phase compensation processor 211 and the vehicle speed V from the vehicle speed sensor 206.
The controller 210 also includes a phase compensation gain setter 216 for setting a phase compensation characteristic curve, which is used in the phase compensation processor 211. More specifically, as the steering torque signal T detected by the torque sensor 205 becomes greater, a torque-adapted gain setter 214 reduces a gain that is set in the phase compensation processor 211 via a multiplier 215, while simultaneously performing a greater-lag phase compensation, i.e., a smaller-advance phase compensation. As the vehicle speed V detected by the vehicle speed sensor 206 becomes lower, a vehicle-speed-adapted gain setter 213 reduces the gain that is set in the phase compensation processor 211 via the multiplier 215, together with performing a greater-lag phase compensation, i.e., a smaller-advance phase compensation.
Such characteristic changes are shown in FIG. 7 of the accompanying drawings. A gain G is increased (shifted) commensurate with changing from a gain characteristic curve L0 to a gain characteristic curve L1, and a phase θ is shifted commensurate with changing from a phase characteristic curve L10 to a phase characteristic curve L11.